Many chemical treatments are available for changing the appearance of hair. For example, chemical treatments for permanently straightening or curling the hair are common. Also, hair may be lightened or bleached and oxidative dyes can be used to change the color of the hair. Chemical treatments are popular because their effects are long-lasting and can be drastic. Nonetheless, the biggest drawback to chemical treatments is the strain and damage caused to hair. This is because chemical treatments permanently change the chemical and physical structure of the hair. Chemical treatments can remove moisture from the surface of the hair cuticles resulting in the hair becoming brittle, dry, and more vulnerable to breakage.
Individuals seeking to change the shape of hair often turn to chemical procedures that use chemical relaxer compositions. Chemical relaxer compositions are often used on curly hair. The chemical relaxer compositions make hair easier to straighten by chemically “relaxing” the natural curls. The active agent is often a strong alkali, although some formulations are based on ammonium thioglycolate instead. Hair relaxer compositions are applied to hair at a salon by a professional or in the home by the individual consumer.
Hair fiber is a keratinous material, which is comprised of proteins (polypeptides). Many of the polypeptides in hair fibers are bonded together by disulfide bonds (—S—S—). A disulfide bond may be formed from the reaction of the two sulfhydryl groups (—SH), one on each of two cysteine residues, which results in the formation of a cystine residue. While there may be other types of bonds between the polypeptides in hair fibers, such as ionic salt bonds, the permanent curling and shape of the hair is essentially dependent on the disulfide bonds of cystine residues.
Chemical relaxing processes alter the aforementioned disulfide bonds between polypeptides in hair fibers to form lanthionine [S(CH2CHNH2COOH)2]. Thus, the term “lanthionizing” is often used when referring to the relaxing or straightening of keratin fibers by hydroxide ions. Hair fibers may be relaxed or straightened by disrupting the disulfide bonds of the hair fibers with an alkaline agent or with a reducing agent. The chemical disruption of disulfide bonds with an alkaline agent is generally combined with mechanical straightening of the hair, such as combing, and straightening generally occurs due to changes in the relative positions of opposing polypeptide chains within the hair fiber. This reaction is generally terminated by rinsing and/or application of a neutralizing composition.
The reaction with the alkaline agent is normally initiated by hydroxide ions. Hair relaxing processes proceed via the release of the hydroxide ions, which penetrate the hair fiber and transform cystine residues to lanthionine residues. Chemical relaxer compositions often contain varying proportions of strong water-soluble bases, such as sodium hydroxide (NaOH), or include slightly-soluble metal hydroxides, such as calcium hydroxide (Ca(OH)2), which can be converted in situ to soluble bases, such as guanidine hydroxide. Sodium hydroxide is extremely effective in straightening hair fibers but often causes a decrease in the strength of the hair fibers. Chemical relaxer composition often damage the hair to an extent and cause the hair to lose some of its desirable cosmetic properties such as shine, strength, smoothness, etc. Thus, mechanisms to reduce or prevent damage to hair and for improving the cosmetic properties of hair treated with chemical relaxer compositions are desired.